Breast cancer incidence rates vary widely among different racial and ethnic groups. Admixed populations, in which two or more ethnic groups have mixed, may provide insight into the etiology of these differences. Latino Americans are largely an admixed group descended from European, Native American, and African ancestors. In California, the majority of Latinos are immigrants from Mexico and Central American countries, and are mainly of European and Native American ancestry. Latinas have breast cancer incidence rates that are substantially lower than Caucasians but higher than Native Americans, consistent with their mixed European and Native American descent. This project will examine the association between genetic ancestry among Latinas and breast cancer risk. Using an established case-control study of Latinas in the Bay Area we will compare genetic ancestry among 240 Latinas with breast cancer and 300 age-matched Latina controls. Ancestry will be estimated with a series of genetic markers highly informative for ancestry. Analyses will be adjusted for known non-genetic risk factors for breast cancer. We will also examine the association between mammographic density and genetic ancestry among Latinas. Mammographic breast density is one of the strongest known risk factors for breast cancer and is a highly heritable trait. To address this aim we will recruit 100 Latina women with extremely high mammographic density and compare the genetic ancestry among these women with 100 Latina women with extremely low breast density. Based on these results we will determine whether genetic association studies of breast cancer risk and of mammographic density may be confounded by ancestry in the Latina population. In addition, this project will test the feasibility of using the increased linkage disequilibrium in admixed populations to identify genetic variations associated with breast cancer among Latinas.